The present invention relates to a process for making a lithographic printing plate having a silver image utilized as ink receptive areas and in particular to a process for making a lithographic printing plate which has no stains and is markedly enhanced in printing endurance of silver image.
There have already been printing plates in practical use which are made using silver halide emulsions having a high sensitivity and capable of being spectrally sensitized. Among them, offset printing plates where silver images are utilized as ink receptive areas are disclosed in U.S. Pat. Nos. 3,721,559, 3,490,905, 3,385,701, 3,814,603, 3,454,398, 3,764,323 and 3,099,209, Japanese Patent Kokoku Nos. 48-30562 and 44-27242 and Japanese Patent Kokai Nos. 53-21602 and 53-9603.
These may be roughly classified to some types of printing plates, but they are common in utilization of silver as ink-receptive areas.
A lithographic printing plate consists of greasy ink receptive oleophilic image portions and ink repellent hydrophilic non-image portions, the latter being generally water receptive hydrophilic areas.
Accordingly, the ordinary lithographic printing is carried out by feeding both water and colored ink to the printing plate surface to allow the image portions to receive preferentially the colored ink and the non-image portions to receive preferentially water and then transferring the ink on the image portions onto a substrate such as paper.
In order to obtain a print of good quality, it is necessary that oleophilicity of the image portions and hydrophilicity of the non-image portions are both strong enough so that when water and ink are applied the image portions may receive sufficient amount of ink while the non-image portions may completely repel the ink and it is desired that this effect can be retained for printing of as many as possible copies.
The process for making printing plate using silver halide emulsion mentioned above is simple and reliable and can be performed at high speed and can be automated and besides the resulting printing plate has a high sensitivity, a high resolution and a high image reproducibility. However, the process suffers from the problem that the resulting printing plates cannot stand printing of a larger number of copies, namely, inferior in printing endurance as compared with other printing plates, for example, PS plates put in practical use which consists essentially of a bichromate sensitized hydrophilic colloid and a diazo sensitized organic colloid.
Many attempts have been made for improving these defects. One of them is use of an oxidizing agent to silver image. For example, Japanese Patent Kokai No. 55-98753 discloses use of an oxidizing agent and a compound having in a molecule a mercapto group or a thion group and a hydrophilic group. Further, Japanese Patent Kokai Nos. 51 58952 and 61-223740 disclose use of an oxidizing agent and an iodide (for example, sodium iodide). However, although these methods can enhance printing endurance of silver image, the enhancement is still insufficient as compared with PS plate and besides, ink receptivity is sometimes deteriorated.
As a result of the inventors' intensive research in an attempt to improve printing endurance by utilizing an oxidizing agent for silver image, it has been found that printing endurance markedly superior to that attained by the above-mentioned conventional methods can be obtained by using a certain kind of compound.